Add ability to create a local cache instance

[lindskogen]

We're generating a lot of cache keys in our app, and would like to clear all keys under a specific "namespace".

1. My first idea was to add the ability to clear keys by prefix, but searching through all the keys seems like a bad idea for perf.
2. My other suggestion is to create a cache specific to each use-case in the app, and a clear function to delete the whole list in that namespace.

This seemed like more in-line with the direction the React team are taking with scoped cache.

We're already using this solution in our production app with great results :)

[lindskogen]

Sorry. I updated the description now!